The Isotron is a real antenna, not a dummy load as some would suggest. Working with the Isotron 200B antenna is quite a learning experience however. If you remember my previous post, I was lamenting about the Isotron’s touchy tuning. First let me say it is a narrow band device and it can be quite unpredictable. I finally tuned the antenna in a horizontal position, with the 10’ mast hanging on two nails on posts of a walk out basement deck and the coil was approximately 5’ above ground. I got the antenna to tune to 1640 kHz @ 1.5:1SWR showing about 8 watts. The range was about 2 miles.

But here is a rule of thumb to remember…when I raised the antenna to vertical position on my 30’ telescoping mast raised to only 11’ my resonant frequency was now raised to 1695 kHz…That’s about 55 kHz. The antenna is about 7’ from top plate to bottom plate and the bottom plate is now about 14’ above ground . The antenna included about 8” of tuning rod extension to the already 36” to tune at 1640 kHz (normal is 1650-1800.) The tuning rod was set to 23” over the top plate. The antenna’s tuning rod was then maxed out to about 32” above the top plate and the resonant frequency was lowered to about 1660 kHz @ 2.4:1 SWR showing about 4 watts. The total height is about 24’ from tip of tuning rod to ground. The range on my car radio was very good all things considered…2 miles very strong local, 5-6 mile distant, and about 8-10 miles fringe. That was yesterday…

It snowed overnight about 4” on the plates of the antenna and was cold this morning and my frequency was holding at 1660, but my SWR went down to 1.8:1, which is good thing. It’s going to warm up midweek and I’ll try to tweak the unit down to 1640 kHz by adding a small circular plate above the coil to raise capacitance. I’ll also try a few things to get a good impedance match to lower my SWR. Then I’ll raise my antenna another 5’ or so.

A word about antenna height, 25 to 30 feet gives you the biggest bang for the buck on frequencies below 2,000 kHz. You really wont get much more range for the trouble. On 100 MHz FM, it is a different story, the higher the better. That’s why high frequency FM antennas are put on a mountain, but low frequency AM antennas are put in a valley along river banks and salt marshes. The only reason that AM antennas are big is a matter of frequency and wavelength. The Isotron solves that issue. Another plus for low power AM is that you can get away with low cost RG-58/U which shows about 0.5 Db loss per 100’ below 2,000 kHz (The ARRL Antenna Book.). I use 50’ Radio Shack coax, and the losses are negligible. You’re better off focusing on resonance and impedance issues first if you want to get out.

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Thanks for the info.. Reference your range, has that improved at all now that you have had some experience tweaking the antenna?

Wed Dec 22, 2004 1:52 pm

erobertgCompulsive poster:)

Joined: 29 Oct 2004Posts: 113

Still Tweaking...

I'm still tweaking my antenna. This morning, I added a 2"x7" metal stub just on top of the coil. It had a opened ended slot so all I had to do is tighten the 1/4-20 nut on it. The SWR now dips to 2.2:1 at 1,640 kHz so now it resonates where I want to. The SWR quickly raises after 1,635 and 1,645 to well over 3 and beyond, so it appears I honed it in. The power meter shows about 5 watts. The range is very good, and I'm burning a lot of gas doing range checks. This baby really gets out...

The next thing to address is impedance issues, get everything 50 ohms...The strange thing is when I disconnect the audio cable the SWR drops to a respectable 1.5...But I think I'll keep my streaming audio.

Maybe PCS Mark might have some ideas?

Wed Dec 22, 2004 9:50 pm

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Quote: "The strange thing is when I disconnect the audio cable the SWR drops to a respectable 1.5"

It looks like your audio cable has become part of your antenna system. On other types of antennas you would use a balun so that you are sending all of your power to your antenna instead of radiating your feed line. Check with the guy at Isotron for his suggestions also make sure you have a good ground.

BTW, it sounds like the band width of your antenna (frequency at which it is resonate on) is very narrow. This is fairly common on "non traditional" type antennas. I have seen 10 to 20 kc many times.

You say it really gets out, just curious how far?

A-man

Wed Dec 22, 2004 10:37 pm

erobertgCompulsive poster:)

Joined: 29 Oct 2004Posts: 113

Range...

The range is at least 2-3 miles strong local, 4-6 distant, 8-10 fringe at 2.2 SWR. Like I said, this at an output of 4-5 watts. I'm using the 19V power supply but I think my antenna impedance is too high. Nice thing about medium band AM, the signal even travels over a range of 1,000' hills.

Still have to raise my antenna 5 os so feet.
Lower my SWR by making my system as close to 50 ohm throughout.

Wed Dec 22, 2004 11:48 pm

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Thanks for the report.

Regarding the impedance of the antenna, I have heard that it is a misconception that lower SWR is always better. As I understand antennas, the feedpoint impedance is and should always be secondary in importance to the "radiation resistance" of the antenna. One of the links below relates antenna effeciancy as a ratio of feedpoint impedance vs. radiation resistance. (bottom of page - hints part one) I have included some links below just in case you want to read up on this stuff.

BTW, the first couple of links include some good general guidlines about verticals. Maybe you can use some of this on setting up your Isotron.

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Yeah,

I would like to have one but have a lot going this month with tuition and taxes. I will probably take the plunge in Jan or Feb. Hopefully the dollar will appreciate a little between now and then.

On the Isotron, I am not sure yet. I have 30' vertical that I have built and am working on the loading coils as I have time. BTW, One of the reasons I am interested in the AM transmitter is for one reasons you mentioned. It should transmitt over the hills better than my current FM transmitter is doing now. I have tried several things on FM including circular polarized antennas and still find areas on my place that the signal drops out.

The AM transmitter will be a hoot to mess around with anyway.

Later, A-man

Thu Dec 23, 2004 6:06 am

erobertgCompulsive poster:)

Joined: 29 Oct 2004Posts: 113

AM MAX II DSP

The AM MAX II DSP is a good transmitter to get, sounds great, full modulation and decent power. The FM band is too crowded and closely monitored by big brother. The only thing that plagues AM is power lines which I think the FCC has done a lousy job monitoring. My signal rides over all that shit the first 2-3 miles. The FCC shouldn't be concerned with some guy cranking out a few watts at 1640 kHz doing a community service play radio shows that aren't commonly available. Haven't had this much fun since is was a kid with a wireless microphone.

Thu Dec 23, 2004 3:14 pm

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Reference your power line noise, I think the power companies are required to fix those problems when they come up. My power company helped me last year when i had some really bad noise. They came out with all sorts of monitoring equipment and went right to the source. You might give it a try....

BTW- Merry Christmas All!!!

Sat Dec 25, 2004 5:31 am

erobertgCompulsive poster:)

Joined: 29 Oct 2004Posts: 113

Power Line Noise...

We had a bad bout with power line noise about two months ago. The guy from the utility company came over and wouldn’t you know, there wasn't any noise that day, but then it came a couple of days later. There is no rhyme or reason to the noise but my neighborhood has been fairly quite lately. But as I drive around there are pockets of noise everywhere. Now there is talk of BPL which is supposed to doom all communications from 2-80 mHz.

Yes, Merry Christmas

Sun Dec 26, 2004 12:39 am

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

Here is a real simple antenna that some may want to try to build. The total cost on this should be around $30 or 40 depending on what materials you may have on hand. There is plenty of opportunity for tweaking if you know what you are doing.

I am currently thinking of building a center fed dipole which would use two of these joined in the center. All I need is some time off to work on it.

The antenna you described is a loaded dipole. I checked the wire used on the Isotron and it is solid 14 AWG house wire. The price is $15.00 for a 500' reel...Not bad. Are you going to have it horizontal? If so, you probably have directional figure 8 pattern.

Fri Dec 31, 2004 1:25 am

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

I was just thinking about building a vertical dipole however, I have a "EE" (genius brother in-law) who convinced me that it should be built with one center coil between the top and bottom halfs, and then be inductively fed. He says that will allow me to set the feed point impedance to whatever i need and also have some safety from lightning strikes.

He has the coils so all I need to provide is the aluminum tubing stock. I will let you know how it works out in a couple of weeks or so. By then I should have my xmiter.

BTW, How is the Isotron working out? Have you got it working the way want yet?

Fri Dec 31, 2004 3:20 am

erobertgCompulsive poster:)

Joined: 29 Oct 2004Posts: 113

Isotron Performance Report Part II

I tried a value of 300 pf between the bottom ground plate and the mast to lower the SWR. Weather can be a good thing, at least as far as antennas are concerned. The last 4 days my SWR has been hovering around 1.7:1. Last night it snowed about 6" and my antenna is holding steady at an even lower SWR. I figure by February my SWR should be well under 1.5:1. Everything is running fine and range is good.

Thu Jan 06, 2005 4:50 pm

AntlerCompulsive poster:)

Joined: 29 May 2004Posts: 90Location: Somewhere between the RJAA and EHAM airports

I have ordered my transmitter and hopefully I will have it in the next week or so. I haven't had time to build an antenna yet but have a vertical cut for another band and will try that with a tunner first. (I hate losing power through a tuner!) It will be fun...

Regards, A-man

Thu Jan 06, 2005 5:11 pm

MOSFETNew registered user

Joined: 26 Feb 2005Posts: 5

Re: Isotron Performance Report Part I

erobertg wrote:

The Isotron is a real antenna, not a dummy load as some would suggest. Working with the Isotron 200B antenna is quite a learning experience however. If you remember my previous post, I was lamenting about the Isotron’s touchy tuning. First let me say it is a narrow band device and it can be quite unpredictable. I finally tuned the antenna in a horizontal position, with the 10’ mast hanging on two nails on posts of a walk out basement deck and the coil was approximately 5’ above ground. I got the antenna to tune to 1640 kHz @ 1.5:1SWR showing about 8 watts. The range was about 2 miles.

I’ll keep you posted in Part II.

Hi. Just some notes on the ISOTRON antenna.
I am not sure if the commercial ISOTRONs come with a HF choke.
You will need an RF choke between the TX and Isotron, or the coax will become a part of the antenna
I am not sure, but you said earlier you had problems tuning the ISOTRON when attaching audio. That sounds exactly like the HF current is going back on the coax, thus becoming a part of the TX and audio gear. Not good.

I have just built my own Isotron from scratch, just looking at pictures and various specs and they are really difficult. The biggest issue is the choke. I have tried two ways.

1. wind about 30 turns of coax on a massive ferrite core and;
2. A 1:1 balun made from Amidon ferrite cores.

Both solutions worked ok, but the SWR went up from 1:1 to about 1.5:1 when I used the choke. And of course. The resonance frequency went up about 500 kHz. So the choke is a very very important part in setting up an Isotron.

The basic idea of the Isotron seems to be small cap hats and large inductors so I made my Isotron with a PVC drain pipe 63mm diameter (3 meters) and two chicken-fence-net caps size 300x600mm. I wound about 230 turns of electrical 1.5 mm cable directly on the PVC tube between the two plates. My Isotron resonates around 1650 kHz. And with a tuning rod you can lower it about 100 Khz.

For me it seem too much with 230 turns on that PVC pipe. Why? Because I think the coil will be so big that is start generating its own capacitance.
I shall make a new ISOTRON with a 150 mm PVC drain pipe. That will lower the turns and make a far more better coil with higher Q.

I shall post a new thread about my projects later on

Regards

Mosfet

Sun Mar 13, 2005 8:35 pm

MOSFETNew registered user

Joined: 26 Feb 2005Posts: 5

Re: One Thing I Forgot To Mention...

erobertg wrote:

The antenna can show quite a bit of reactance. The Isotron has a characteristic impedance of 20-200 Ohms depending on environment.
This obviously can cause a mismatch to a 50 ohm transmitter.